Re: 4.14.9 with CONFIG_MCORE2 fails to boot

[Linus Torvalds]

On Fri, Dec 29, 2017 at 1:50 PM, Alexander Tsoy <alexander@xxxxxxx> wrote:
>>
>> Ho humm. What happens if you change the "-march=core2" to
>> "-mtune=core2"? Does it still boot?
>
> That's interesting. Compiled with -mtune=core2, the kernel fails to
> boot.

[ Insert "twilight zone" theme music ]

Damn. I was hoping that "-march=core2" would enable something specific
that causes the failure, and that "-mtune=core2" would just schedule
for core2 but not fail, and then we could compare the two and see what
triggers things.

But apparently no such luck. It's apparently just fundamentally the
instruction scheduling and selection for core2 that causes problems,
so mtune ends up being the same as march.

It could be something entirely random, and some instruction scheduling
detail just ends up showing it by happenstance.

And sadly, we have almost nothing to go by.

The fact that double faults seem to be implicated does make me want to
try to disable that ESPFIX64 code in the #DF handler.

What happens if you take a failing kernel, and then in
arch/x86/kernel/traps.c do_double_fault(), you change the

  #ifdef CONFIG_X86_ESPFIX64

to just a

  #if 0

do you then get an actual double-fault oops report instead of the
stall (and NMI oops)?

But honestly, I'm just throwing random ideas out now.

Hopefully somebody else has a better idea than I do. Andy?

                   Linus